1. Field of the Invention
The present invention relates generally to an improvement in sails, and methods of fabricating and utilizing same. In particular, the present invention relates to an adjustable sail head tensioning system preferably, but not necessarily, for a windsurfing sail.
2. Description of the Relevant Art
The term "sail" as used herein is intended primarily to mean a windsurfing sail, but the present invention may also be applicable to sails for other types of vehicles and craft which derive motive power at least partially from the wind.
Most windsurfing sails are provided with a sleeve which is pulled over the mast. In addition, most windsurfing sails are provided with a substantially loose foot. In the early years of windsurfing, the windsurfing sail was substantially triangularly-shaped.
A windsurfing sail is subjected to much greater wear than those of other sailing craft because it is often dropped into the water and also gets rough treatment on the beach or shore.
In any event, a primary concern is to make certain that the sail is controllable and manageable to sail in a great variety of wind conditions and water conditions. The area of some windsurfing sails is approximately 59 square feet. Instead of reducing the area in strong winds by reefing, in the past sometimes a smaller sail was used, such as a 48 square foot all-weather, or all-round, sail which is cut specifically for use when the wind strength increases.
Sometimes a storm sail of approximately 37 square feet is designed for really hard winds, but can also be used by children or by learners when the wind is moderate. There have also been extra-large sails of approximately 70 square feet or greater for use when racing and in light weather, but sail size is invariably limited by the length of the mast and the length of the wishbone boom.
Those sails which are cut with a rounded convex leech have battens to support the trailing edge. The battens are intended to ensure that the sail remains "stiff" and maintains the "best" profile.
The amount of twist in a sail is also a very important consideration. The twist in the sail may be observed by looking from behind the sail when it is sheeted in to see how straight the leech is. If there is a great deal of twist in the sail, the top third of the leech will fall away. On the other hand, if the leech is too tight, it will hook the top part of the sail to windward. To better understand the amount of twist in a sail, one must consider wind speed.
Although the wind speed arising from the windsurfer board speed is substantially the same at all heights from the board to the top of the mast, the true wind speed increases with height above the surface of the board. The wind approximately four yards above the board blows about 25% faster than the wind one yard over the board. For example, a wind which is blowing at about 16 knots at a height of one yard above the board will blow at about 29 knots at the head of the sail. Consequently, when the board is sailing at 10 knots on a beam reach the apparent wind gradually frees to blow from a direction of about 5.degree. nearer the stern at the head of the sail. When the board is sailing at the same speed in such a wind on a close-hauled course the difference in the angle of attack is only about 2.degree., but when broad reaching is about 12.degree..
Heretofore, sail camber was varied from the sail foot to the sail head to match the gradual freeing of the wind with height above the board. When a sail which is full of wind is viewed from astern, it is evident that the upper third of the leech must be open in a wind of this strength.
When the sail or the leech is said to be "open", it means that the horizontal profile of the sail is flat near the leech which, owing to wind pressure, lies to leeward of the direct line between the sail head and the sail clew. If the leech or the sail is said to be "closed", the curvature of the sail is continued right to the leech which stands up to windward of this direct line. The art of sail trimming is to adjust the tension of the leech so that the amount that it opens matches the wind at the moment. When close-hauled, the leech should not be closed by wind pressure. Rather, it must lie to leeward of the line from the head to the clew.
Previous techniques for attempting to adjust the tension of the leech involved varying the mast curvature, the foot tension, and the height at which the wishbone boom is attached to the mast. Other attempts involved the use of a kicking strap which could therefore be called a leech tensioner.
Raising the front of the wishbone boom makes the outhaul pull downwardly more, which in turn tightens the leech of the sail, thereby reducing the amount of twist. However, most sails do not allow much adjustment of the wishbone boom height. Furthermore, it is uncomfortable to sail with a boom which is too high or too low.
Another way of attempting to control the leech tension is to use a kicking strap or boom vang, that is, a line from the foot of the mast to the end of the boom. This can be very effective, but it does have disadvantages. One disadvantage is that the boom vang tends to catch on the top of the daggerboard when tacking. Also, as the mast bends, the vang slackens off and loses its effect. Thus, the kicking strap or boom vang is not very beneficial with a bending mast or in gusty winds. Overall, the use of a kicker or boom vang is not recommended.
None of the previous devices and techniques have provided a construction, assembly or system which easily controls the tension on the sail head and the upper leech of the sail. Indeed, a desideratum of the present invention is to avoid the animadversions of the previous devices and techniques. It would thus be desirable to provide a construction and system which, in addition to eliminating the aforementioned problems and disadvantages of the previous techniques, provides very new and desirable features heretofore unattainable.